In some semiconductor devices having deep trenches formed therein, it is required to form a patterned film both on bottom of the deep trenches and over the substrate. For example, in a Z-direction sensor of a three-dimensional micro-electromechanical systems (MEMS) device, as shown in FIG. 1, patterned magnetic films 601 and 602 are needed to be formed respectively on bottom of the deep trenches 200 and over the substrate 100. FIGS. 2a and 2b are photos respectively showing the results of the conventional one-step photolithographic processes using respective positive (FIG. 2a) and negative (FIG. 2b) photoresists. As illustrated in the photos, due to a limited depth of focus (DOF) and the absorption of the exposure light by the photoresist during the exposure process, neither the positive nor the negative photoresist approach could ensure a sufficient light intensity for photoresist coated on the bottom of the deep trench, such that it is impossible to form a desired photoresist pattern on the bottom of the deep trench after the development process. Thus, this approach is not able to simultaneously forming the desired magnetic film patterns respectively on bottom of the deep trenches and over the substrate in a subsequent etching process.
The existing method for addressing the above-mentioned issues includes two photolithographic processes and two etching processes. The method includes the following steps: 1) performing a first photoresist coating, exposure and development process to form a first desired photoresist pattern over a magnetic film which is formed in advance on the bottom of the deep trench 200; 2) forming the magnetic film pattern 601 covering part of the bottom of the substrate 100 (see FIG. 1) by etching the underlying magnetic film using the first photoresist pattern formed in step 1) as a mask; 3) performing a second photoresist coating, exposure and development process to form a second desired photoresist pattern over a magnetic film which is formed in advance over the substrate 100; and 4) forming the magnetic film pattern 602 covering a top surface of the substrate 100 (see FIG. 1) by etching the underlying magnetic film using the second photoresist pattern formed in step 3) as a mask. Although this method is capable of addressing the above-mentioned issues, the adoption of two photoresist coating processes, two exposure processes (which need two masks during the two exposure processes), two development processes and two etching processes leads to high complexity and cost.